March 4, 2018. Emergency services find an unconscious Sergei and Yulia Skripal on a bench in downtown Salisbury. They are transported to the hospital where doctors struggle to find the cause of the disease. DS Nick Bailey has to stop his investigation when he feels sick too. Tracy Daszkiewicz, the local director of health care, attends an emergency meeting that reveals an assassination attempt involving an unknown poison. Tracy soon becomes aware of the potential danger and decides to close down the key locations, only to find that the entire city center could be contaminated. As Tracy grapples with the responsibility of the safety of thousands of civilians, Nick's health continues to deteriorate.